1. Field of the Invention
The present invention relates to a backlight unit. More particularly, the present invention relates to a backlight unit which permits an LCD device to realize five primary colors using only a four sub-pixel structure, an LCD device having the backlight unit, and a method of driving the LCD device.
2. Discussion of the Related Art
A Cathode Ray Tube (CRT) has been widely used for computer monitors and televisions. However, the CRT is heavy and bulky. Thus, display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP) and an Electroluminescence display (ELD), have been substituted for the CRT. Among the alternative display devices, the LCD device has been very popular due to the LCD device having a low power consumption, thin profile and lightness in weight. The LCD device has been employed as a monitor for a desktop computer, a large sized television display device, as well as for a laptop computer.
The LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel has first and second glass substrates bonded to each other with a predetermined interval therebetween, and a liquid crystal layer injected between the first and second glass substrates.
On the first glass substrate (TFT array substrate), there are a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines in a matrix, and a plurality of thin film transistors (TFTs), switchable in response to signals on the gate lines, for transmission of signals on the data lines to the pixel electrodes.
The second glass substrate (color filter substrate) has a black matrix layer for shielding light from areas excluding the pixel regions, a color filter layer (R, G, B) for displaying colors, and a common electrode for implementing a picture image.
The foregoing first and second glass substrates have a predetermined gap therebetween which is maintained by spacers, and the first and second glass substrates are bonded by a sealant having a liquid crystal injection inlet. Liquid crystal is injected through the liquid crystal injection inlet. The LCD device controls transmittance of ambient light to display the picture image. In this respect, the LCD device requires an additional light source, such as a backlight. The backlight is classified into a direct type backlight and an edge type backlight, according to a position of lamps.
In the edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit is provided with a lamp emitting light, a lamp holder holding both ends of the lamp to protect the lamp, and a reflective plate having one side facing the light-guiding plate and partially surrounding the circumference of the lamp, to reflect the light emitted from the lamp to the light guiding plate. The edge type backlight unit is generally applied to relatively small sized LCD devices, such as the monitor of a laptop computer or a desktop computer. The edge type backlight is advantageous in that it has great uniformity of light, long life span, and a thin profile.
The direct type backlight unit is suitable for a large sized LCD device of 20 inches or more. In the direct type backlight unit, a plurality of lamps are arranged in one direction below a light-diffusion plate to directly illuminate an entire surface of the LCD panel with light. The direct type backlight unit has great light efficiency and is commonly used for the large sized LCD device requiring high luminance. A large-sized monitor or television of the LCD type would use the direct type backlight unit. After the lamps are driven for a long time, there is a tendency for the lamps to malfunction or simply burn out at the end of the lamp's life. In the direct type LCD device, a plurality of lamps are formed underneath a screen. Accordingly, if one of the lamps turns off due to malfunction or the end of the lamp's life, a screen portion corresponding to the turned-off lamp becomes darker, as compared to surrounding portions of the screen. In this respect, the direct-type LCD device has to have a simple structure suitable for disassembly and reassembly of the lamp unit.
LCD devices can use light sources such as Electro Luminescence (EL), a Light Emitting Diode (LED), a Cold Cathode Fluorescent Lamp (CCFL) or a Hot Cathode Fluorescent Lamp (HCFL). The CCFL has a long lifetime, low power consumption and a thin profile is commonly used as the light source for a large sized color TFT LCD device.
In case of the CCFL, a fluorescent discharge tube is used for using a penning effect, which is formed by injecting a hydrargyrum gas containing Argon (Ar) and Neon (Ne) at a low temperature. Electrodes are formed at both ends of the fluorescent discharge tube, and the cathode is formed in a plate-shape. When a voltage is applied thereto, electric charges inside the fluorescent discharge tube collide against the plate-shaped cathode, thereby generating secondary electrons. Circumferential elements are excited by the secondary electrons, whereby plasma is generated. The circumferential elements emit strong ultraviolet rays, and then the ultraviolet rays excite a fluorescent substance, thereby emitting visible rays.
The aforementioned LCD device controls the light transmittance on the screen with the liquid crystal, thereby determining colors and brightness on the screen of the LCD device. Thus, the LCD device has the following characteristics: varying a viewing angle varies a picture quality according to an angle of a viewer's eye to the screen, a light transmittance according to a transmissive light-emitting display, a color realization ratio by realizing red(R), green(G) and blue(B) colors by transmitting the light through a color filter layer, a luminance for the brightness of the picture image, and a residual image remaining on the screen after one picture image is displayed on the screen for a long time.
Currently, LCD device has some disadvantages to the color realization ratio and the luminance ability as compared to a CRT. The related art LCD device of a notebook computer has a color realization ratio of 40% to 50% as compared with an NTSC method adopted for color television broadcasting by the National Television System Committee. For a computer monitor, this is enough to satisfy the viewers. However, in the case of television, this color realization needs improvement. It would be desirable to develop an LCD device having a greater color realization ratio than that of the CRT.
A general multi-color LCD device includes an LCD panel, a backlight and a color filter. The general multi-color LCD device uses the backlight of a fluorescent lamp having three wavelengths as a light source. A white light emitted from the backlight is divided into three colors (red, green and blue) in the color filter, and the divided colors are mixed again to display various colors.
The color of the light source is determined according to chromaticity coordinates of the Commission International De L'eclairage (C.I.E.). That is, tristimulus values ‘X’, ‘Y’ and ‘Z’ are calculated from a spectrum of a predetermined light source, and then x, y and z chromaticity coordinates of red, green and blue are calculated according to a conversion matrix. Subsequently, x and y values of the red, green and blue are expressed as rectangular coordinates, so that a U-shaped spectral locus is drawn, which is called as CIE chromaticity diagram. The general light source has the chromaticity coordinates inside the U-shaped spectral locus. A triangle space of the red, green and blue chromaticity coordinates becomes a color realization space. As the triangle space becomes large; the color realization ratio becomes greater. The color realization depends on color purity and luminance. As the color purity and the luminance become greater, the color realization increases. Herein, the tristimulus values ‘X’, ‘Y’ and ‘Z’ indicate a weight of a color-matching function approaching to one spectrum. Especially, ‘Y’ is a stimulus value to the brightness.
A color temperature means a temperature of the hue of the white color according to the color change of the light emitted by a temperature of a heat source. On a monitor, the color temperatures appear as 9300K, 6500K and 5000K. As the color temperature becomes close to 9000K, the hue of the white color contains a blue color. When the color temperature is 6500K, the hue of the white color contains a red color. When the color temperature is 5000K, a neutral hue is generated. The color temperature is obtained from the chromaticity coordinates (x, y) of the white color. As the color temperature becomes close to 9000K, it satisfies European broadcasting union (EBU) standards.
In case of the aforementioned LCD device, a luminous spectrum of the backlight is coupled with the color-matching function and a transmission spectrum of the color filter to determine the tristimulus values at each wavelength of the visible ray region. That is, in order to obtain the various colors, it is required to control a correlation between the backlight/color filter and the tristimulus values. In other words, the luminous spectrum of the backlight has to be controlled to optimize the color realization and the color temperature, and the transmission spectrum of the color filter has to be controlled to optimize luminosity.
To overcome the limit of recent color realization ratios of the LCD device having three primary colors (R, G, B), it has been proposed to develop an LCD device realizing four or five primary colors including yellow and/or cyan color as well as R, G, and B colors.
Hereinafter, a backlight unit of an LCD device according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view of a direct type backlight unit of an LCD device according to the related art. As shown in FIG. 1, the backlight unit according to the related art includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c. The outer case 3 fixes and supports the plurality of fluorescent lamps 1, and the light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown). The light-scattering means 5a, 5b and 5c prevent the silhouette of the fluorescent lamps 1 from being visible on a display surface of the LCD panel (not shown), and provide a light source with uniform luminance. For improving the light-scattering effect, the light-scattering means includes a diffusion plate 5a, a prism sheet 5b, and a protection sheet 5c. Also, a reflective sheet 7 is provided inside the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel.
Each fluorescent lamp 1 is a Cold Cathode Fluorescent Lamp (CCFL). Electrodes are provided at both ends of each CCFL, whereby the CCFL emits light when power is provided to the electrodes. Both ends of the fluorescent lamp 1 are fixed to both sides of the outer case 3. Furthermore, power supplying wires 9 and 9a are respectively connected to the electrodes provided at both ends of the fluorescent lamp 1 to transmit power to the fluorescent lamp 1. The power supplying wires 9 and 9a are connected to a driving circuit by additional connectors.
A method of driving an LCD device with five primary colors will be described in brief. FIG. 2 is an array plane view of a color filter layer in an LCD device using five primary colors according to the related art. FIG. 3 is a plane view of a direct type backlight unit applied to an LCD device using five primary colors according to the related art.
To drive an LCD device with five primary colors with a direct type backlight unit, as shown in FIG. 2, a color filter layer is provided to have a unit pixel including five sub-pixels of red (R), green (G), blue (B), yellow (Y) and cyan (C). That is, the color filter layer is formed to have five primary colors. As shown in FIG. 3, the direct type backlight unit is provided to illuminate the LCD device with the five primary colors. The lamps 31 used for the backlight unit are Cold Cathode Fluorescent Lamps (CCFL), wherein the CCFL is made using a method to form the color peak with pigment.
In order to have the LCD device with five primary colors, it is necessary to provide the pigment of a desired color (for example, yellow or cyan) to the lamp of the backlight unit. However, it is difficult to control the composition ratio with R, G, and B pigments.
In addition, a CCFL of the five primary colors is not common in the art. Therefore such a CCFL would be costly to fabricate and employ.
As seen in FIG. 2, one pixel is provided with five sub-pixels. This lowers the aperture ratio and luminance.